The present invention relates to electromagnetically actuated valves and particularly valves of the type employed for controlling flow in a conduit, such as fuel vapor flow from an automotive fuel tank vapor canister to the engine air induction manifold or inlet.
In the control of automotive emissions, it has become necessary to trap fuel tank vapors in a canister, typically filled with granular carbon or charcoal, during periods of engine operation and inoperation. Upon engine start-up, fuel tank vapor is directed to the engine air induction inlet to purge the canister. In order to properly control the flow of fuel vapor from a canister to the engine air inlet during operation, it is necessary that the fuel vapor flow be controlled according to the engine operating conditions and induction air flow to prevent unduly rich fuel mixtures. Heretofore, fuel vapor flow to the engine inlet has in some cases been controlled by control valves operating with engine manifold depression or vacuum employed as a control signal acting on a pressure responsive diaphragm for controlling movement of a valve obturator.
An electrically operated valve has also been employed to control an atmospheric bleed to create a pressure differential for causing the diaphragm to move the valve obturator to control vapor canister purge flow. This technique has been found to have limitations; namely, that for a valve calibrated for normal engine operating flow, under certain conditions such as elevated ambient temperatures vapor pressure in the canister could overcome the force of the diaphragm preload spring and force the valve open during engine shut down. In addition, increases in fuel vapor pressure in the system canister due to high ambient temperatures can cause the valve diaphragm to open the valve more than desired during engine operation which has resulted in overly rich engine fuel-air mixtures, and in some cases engine stalling. Accordingly, it has become desirable to electrically directly control a vapor flow valve for fuel vapor canister purging. It has been desired to provide control of such fuel vapor canister purge flow from a the on-board engine microcontroller according to an algorithm based upon measured engine operating parameters in order to adequately control the flow of vapor to the engine air inlet and prevent an unduly rich mixture and improper engine operation.
Heretofore, electrically operated flow valves employed to control fuel vapor canister purge have employed linear type solenoids. Linearly acting solenoid valves have been subject to sliding friction of the armature which has caused excessive hysteresis. Typically, such valves employ the armature to move an elastomeric valve member for opening and closing against a valve seat. Swelling of the elastomeric material when exposed to fuel vapors and the compression set of the elastomeric material when held against the valve seat or sealing surface for extended periods of time have caused problems in control and the service life of such valves during exposure to the elevated temperatures encountered in motor vehicle engine compartment applications. An example of such a known linear solenoid electrically operated valve is that described in U.S. Pat. No. 5,551,406.
It has thus been desired to provide a low cost, reliable, precision flow control valve for controlling fuel vapors from a canister in a manner which will maintain the proper operation of the engine over the desired speed and load envelope and do so reliably and repeatedly for an extended service life and continue to meet mandatory fuel vapor emission limits.
It has also been desired to provide an automotive fuel vapor canister purge control valve which is resistant to opening under elevated ambient temperatures encountered during periods of engine inoperation which would permit fuel vapor escape through the engine air inlet to which the valve is connected.